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A Fluorine-Free Slippery Surface with Hot Water Repellency and Improved Stability against Boiling Ryo Togasawa, Mizuki Tenjimbayashi, Takeshi Matsubayashi, Takeo Moriya, Kengo Manabe, and Seimei Shiratori ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b15689 • Publication Date (Web): 11 Jan 2018 Downloaded from http://pubs.acs.org on January 11, 2018

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ACS Applied Materials & Interfaces

A Fluorine-Free Slippery Surface with Hot Water Repellency and Improved Stability against Boiling Ryo Togasawa, † Mizuki Tenjimbayashi, † Takeshi Matsubayashi, † Takeo Moriya, † Kengo Manabe, † and Seimei Shiratori*,†

†

Center for Material Design Science, School of Integrated Design Engineering, Keio University,

3-14-1 Hiyoshi, Kohoku-ku, Yokohama, Kanagawa 223-8522, Japan

[email protected]*

KEYWORDS slippery surface, fluorine-free, hot water repellency, boiling stability, OH-π interaction

ABSTRACT

ACS Paragon Plus Environment

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Inspired by natural living things such as lotus leaves and pitcher plants, researchers have developed many excellent antifouling coatings. In particular, hot-water repellent surfaces have received much attention in recent years because of their wide range of applications. However, coatings with stability against boiling in hot water have not been achieved yet. Long-chain perfluorinated materials, which are often used for liquid-repellent coatings owing to their low surface energy, hinder the potential application of antifouling coatings in food containers. Herein, we design a fluorine-free slippery surface that immobilizes a biocompatible lubricant layer on a phenyl-group-modified smooth solid surface through OH–π interactions. The smooth base layer was fabricated by modification of phenyltriethoxysilane through a sol–gel method. The π electrons of the phenyl groups interact with the carboxyl group of the oleic acid used as a lubricant, which facilitates the immobilization on the base layer. Water droplets slid off the surface in the temperature range from 20°C to 80°C at very low sliding angles (< 2°). Furthermore, we increased the π electron density in the base layer to strengthen the OH-π interactions, which improved long-term boiling stability under hot water. We believe that this surface will be applied in fields in which the practical use of antifouling coatings is desirable, such as food containers, drink cans, and glassware.

INTRODUCTION

Surfaces that can repel hot liquids have attracted much attention because there is a lot of demand for their application in our daily lives and in industry, such as in food containers,1 electrosurgical instruments,2 engines,3 and hot-water pipelines.2 In particular, there can be a significant waste of


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hot liquid food such as retort curry or drinks as residue in food containters.4 This waste results from adhesion of liquid food to inside of the food packages and prevents the recycling of the containers. To achieve repellency of hot foods, hot water repellency as well as stability against boiling are required. Although many liquid-repellent surfaces have been developed, there are very few investigations of coatings with hot water repellency and boiling stability. Moreover, materials used for such coatings must be safe, even if they are consumed. Especially long-chain perfluorinated compounds, which are often used for liquid-repellent surfaces, have been concerned about the influence on the human body.5-7 Liquid-repellent coatings which have previously reported can be broadly divided into two categories based on the difference in their outermost surface morphology; rough or smooth coatings. The most important examples of the former are superhydrophobic surfaces.8,9 Inspired by lotus leaves, superhydrophobic surfaces have a rough hierarchical structure that can hold air to achieve a Cassie–Baxter state. Generally, the surfaces are chemically modified by fluorine materials to accomplish contact angles larger than 150° and sliding angles smaller than 10°. However, they show remarkably limited repellency to hot water because the vapor from hot water induces the transition from a Cassie–Baxter state to a Wenzel state.1,10,11 In recent years, some superhydrophobic surfaces with hot water repellency have been reported.2,12-14 In particular, Li

et

al.

prepared

a

superhydrophobic

surface

by

combining

palygorskite

and

1H,1H,2H,2H-perfluorodecyltriethoxysilane, which repelled hot water at 94°C.2 However, their coating may not be suitable for food packages, because the surface was not biocompatible owing to the long-chain perfluorinated compound. In addition, there are very few papers which investigated stability of superhydrophobic surfaces in harsher situations, such as stability against long-term boiling in hot water.


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One type of liquid repellent coatings with flat surfaces are liquid-infused surfaces (LIS), such as slippery liquid-infused porous surfaces (SLIPS) inspired by Nepenthes pitcher plant.15 SLIPS have a porous base layer for immobilizing a lubricant layer that is immiscible with the target liquids and has good affinity to the base layer; thus, the liquids can easily slide off. The surfaces of SLIPS are smooth lubricant layers so they are hardly affected by the vapor of hot liquids. In fact, Daniel et al. demonstrated that their SLIPS were capable of repelling hot water at 95°C.16 Moreover, there are other examples of SLIPS which demonstrate stability at high temperature.17,18 In addition, since various combinations of hydrophobic base layers and lubricant layers are feasible for SLIPS, some biocompatible SLIPS have been developed.19,20 However, it is reported that sliding speed of target liquids decreased with increasing of an area fraction of a base layer.21,22 Therefore, practical use of SLIPS as liquid-food containers will face a problem of slow sliding speeds of the target liquids, especially liquid foods with a high viscosity, without any improvement. Another example of smooth coatings is liquid-like surfaces, which are fabricated by chemical modification of lyophobic molecule chains through a sol–gel method or layer-by-layer method.3,23-25 In particular, C. Urata et al. developed a thermally durable polymethylsilsesquioxane film, focusing on that all the chemical bonds (Si–C, Si–O, and C–H) present in the films had thermal durability.3 Therefore, oil repellency at 250°C was achieved even without using any fluorine material. Furthermore, liquid-like surfaces have excellent mechanical durability owing to their smooth surface structure. However, it was relatively difficult to slide polar liquids at low sliding angles, compared with their excellent sliding properties for organic liquids. In recent years, a few surfaces which exhibit low sliding angles against both polar and apolar liquids have achieved.26,27 Nevertheless, neither hot water repellency nor boiling stability of the liquid-like surfaces has been reported.


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In 2016, our group developed surfaces with π-interaction liquid adsorption, smoothness, and hydrophobicity (SPLASH) to achieve high-speed removal of water droplets on LIS.28 The smooth base layer of SPLASH was prepared through a sol–gel reaction of phenyltriethoxysilane (PTES) and a lubricant layer was immobilized on the base layer through CH–π interactions. The smooth base layer made it possible to slide off water droplets faster than from a LIS with a rough base layer, because the fluidity of the lubricant layer was less impeded thanks to the smoothness of the base layer.21 Because the outermost surface of SPLASH covered with a lubricant layer was also smooth, SPLASH do not lose their hydrophobicity when exposed to vapor from hot water. In addition, the base layer is thermally stable because it consists of Si–C bonds, Si–O bonds, and benzene rings. Therefore, SPLASH should have a potential to achieve boiling stability. Furthermore, because SPLASH utilize interactions with π electrons instead of a rough structure to immobilize lubricants, a wide range of lubricants can be used. Herein, we fabricated a fluorine-free coating with hot water repellency and improved stability against boiling by utilizing the mechanism of SPLASH. In this study, we chose oleic acid as a lubricant because its –COOH groups interact with the π electrons in the base layer through OH–π interactions.28-30 Furthermore, to improve boiling stability, we controlled the density of the phenyl groups by not using tetramethoxysilane (TMOS) and enhanced the interactions between the lubricant and the base layer. Through the changes described above, we successfully fabricated the fluorine-free surface which slides off hot water droplets at very low sliding angles (< 2°) and exhibits improved boiling stability. We believe this surface will be utilized in various fields and will be especially applicable to coatings for food containers, drink cans, and glassware.


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EXPERIMENTAL SECTION Materials. PTES (Shin-Etsu Chemical Co., Ltd., Tokyo, Japan), ethanol (EtOH / Kanto Chemical Co. Inc., Tokyo, Japan), deionized water (H2O / Aquarius GS-500.CPW, Advantec, Japan), and hydrochloric acid (HCl / Kanto Chemical Co. Inc., Tokyo, Japan) were used for the preparation of a precursor solution (Mixture A). TMOS (Junsei Chemical Co., Ltd., Tokyo, Japan) was used to prepare another precursor solution (Mixture B) for comparison. Glass (76 × 26 mm, thickness: 1.0 mm, refractive index: 1.52, Matsunami Glass Ind., Ltd., Kishiwada, Japan) and a commercial aluminum coffee can with an epoxy resin surface (Daiwa Can Co., Ltd., Tokyo, Japan) were used as substrates. Methyl orange (Junsei Chemical Co., Ltd., Tokyo, Japan) was used as dye for a water droplet to easily observe its movement. Oleic acid (Tokyo Chemical Industry Co., Ltd., Tokyo, Japan) was utilized as a lubricant and oil red (Wako Pure Chemical Industry Ltd., Osaka, Japan) was used as dye for the oleic acid in the boiling tests to identify the area that retained the lubricant after the tests. Canola oil, olive oil, sesame oil, grape seed oil, and linseed oil were purchased from a supermarket. Formamide (Tokyo Chemical Industry Co., Ltd., Tokyo, Japan) and hexadecane (Wako Pure Chemical Industry Ltd., Osaka, Japan) were used to measure the contact angle and surface energy.

Preparation of the solutions. Mixture A, which was used for modification of the surface with phenyl groups, was prepared by mixing 4.452 g of PTES, 5.97 g of EtOH, 0.500 g of H2O, and 0.3 µL of HCl in that order. The molar ratio of Mixture A was PTES/EtOH/H2O/HCl = 1:7:1.5:2.7×10-4, which was the same molar ratio as that reported for the use of methyltriethoxysilane instead of PTES.3 For comparison, Mixture B, which was used to modify the surface with a lower density of phenyl groups, was prepared by mixing 0.4322 g of PTES,


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1.3791 g of TMOS as a cross-linker, 4.41 g of EtOH, 0.7566 g of H2O and 2.039 µL of HCl in that order, according to our previous paper.28 Both Mixture A and B were stirred for 1 day.

Film fabrication. A schematic image of the fabrication procedure is shown in Scheme 1. We used two types of substrates; glass and a commercial aluminum coffee can with an epoxy resin surface. Only the epoxy substrate was first washed by ultrasonic cleaning with deionized water and then with EtOH for 5 min. All the substrates were made hydrophilic by using an ultraviolet ozone (UV/O3) treatment machine (NL-UV253, Shoko Scientific Co., Ltd., (formerly Japan Laser Electron), Kanagawa, Japan). Mixture A or Mixture B were cast on the substrate by spin coating at a speed of 1000 rpm for 5 s and then at 2000 rpm for 10 s. These coatings were dried for 1 day at room temperature and then became the TMOS-free base layer and the TMOS base layer. After drying, 10 µL of oleic acid as a lubricant was cast on the EtOH-rinsed base layers by spin coating at a speed of 1000 rpm for 5 s and then at 4000 rpm for 10 s.

Characterization. The surface morphology of the base layers was analyzed by field-emission scanning electron microscopy (FE-SEM, S-4700, Hitachi, Japan) and atomic force microscopy (Nanoscope IIIa, Digital Instruments, United States). X-ray photoelectron spectroscopy (XPS / JPS-9010TR, JEOL, Tokyo, Japan) with a MGKα laser was used to investigate the chemical composition of the surface and to confirm the presence of π electrons. Static contact angles and sliding angles were measured using a contact angle meter (CA-DT, Kyowa, Tokyo, Japan). The hückel charge distribution was calculated by extended hückel method with ChemDraw (HULINKS Inc., Tokyo, Japan).31 The surface tension (γs) of each base layer and its apolar (γsLW) and polar component (γsAB) were calculated from the contact angles of water, formamide, and


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hexadecane by using the Van Oss method.32 The surface tensions and the viscosities of the lubricant candidates were investigated by an automatic surface tensiometer (CBVP-Z, Kyowa, Tokyo, Japan) and a viscometer (DV1M, EKO, Tokyo, Japan), respectively. The densities of the lubricant candidates were calculated from their mass and volume. The sliding speeds of water droplets were measured by a high-speed camera (HAS-D3, Direct, Tokyo, Japan) and analyzed with Image J software (U. S. National Institutes of Health, Bethesda, Maryland, USA). In all sliding speed tests, 10 µL droplets were carefully placed on the samples. Transmittance measurements in the spectral range of 300 to 800 nm were conducted using a spectrophotometer (UVmini-1240, Shimadzu, Kyoto, Japan). Total transmittance (T.T.), Parallel transmittance (P.T.), diffusion (DIF) and haze values (HAZE) of the surface were measured by a haze meter (NDH-5000, Nippon Denshoku Industries, Tokyo, Japan) with a white light-emitting diode (5V, 3W) as an optical source. The T.T. was the sum of P.T. and DIF. The HAZE was calculated as (DIF/T.T.) × 100.33 The stability against pH change was analyzed by measuring the contact angles and sliding angles of water droplets at various pH values from 1 to 14. Thermal durability was investigated by putting the sample on the temperature-controlled hotplate and measuring the water contact angles and water sliding angles on the sample at various temperatures from 30 to 120°C. The durability against shearing stress by spinning at a speed of ~6000 rpm was investigated by measuring the water sliding angle. In this test, the thickness of the oleic acid was also investigated by measuring the difference in the mass of the sample before and after spinning by using an electric balance. The stability test against flowing water was conducted by flowing 0.32 mL/sec.mm2 of water to a 15° tilted sample from a height of 1 cm. Moreover, evaporation residue measurements were conducted for the TMOS-free base layer fabricated on the epoxy substrate. Heptane (2 mL per 1 cm2 of sample surface area) was utilized as a leaching solution.


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After the samples had been immersed in the leaching solution at 25°C for 1 h, the remaining solution was condensed and evaporated to dryness on an evaporation dish. The amount of evaporation residue was calculated from the weight the evaporation dish after the evaporation experiment (a mg) and the weight of the evaporation dish after evaporation of a blank sample containing the same amount of leaching solution (b mg), using the equation: Evaporation residue (µg/mL) =

×

(1)

The volumes of the water droplets in the above tests were all 10 µL.

Hot water repellency test. We measured the contact angles and the sliding angles of 10 µL water droplets at various temperatures from 20 to 80°C. The temperature of the water droplets was controlled by our hand-made instrument (Figure S1, Supporting Information) and the real temperature of the water droplet on the fabricated surface was measured by thermography (PI400, Optris, United States).

Boiling and retort test. 70 mL of water was put in each of two containers with the volume of 195 mL and heated until the water boiled. Then, samples with the lubricant dyed with oil red were immersed into the boiling water for 30 min. We defined the boiling test as a test in an open container. Conversely, the retort test was defined as a test in a closed container to obtain a high pressure. The contact angles, sliding angles, and sliding speeds of 10 µL water droplets were measured both before and after the tests.


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Scheme 1. Schematic diagram of the preparation procedure and the OH–π electron interactions.

RESULTS & DISCUSSION Surface analysis of the base layer. First, we analyzed the surface properties of the base layer to confirm that the immobilization does not depend on the roughness of the base layer. The SEM image (Figure 1a) and the AFM image (Figure 1b) show the base layer fabricated on a glass substrate using Mixture A for TMOS-free base layer. The fabricated base layer was smooth as shown in the SEM image, though some projections, which seemed to be caused by the aggregation of PTES molecules, were observed in the AFM image. The root-means-square (RMS) roughness calculated by AFM was less than 5 nm. Figure S2 shows the SEM image and AFM image of a base layer coating on an epoxy substrate that had been hydrophilized by the UV/O3 treatment. The SEM images in Figure S2 show that the surface of the epoxy substrate after coating with a base layer was smoother than before coating. However, as indicated in the AFM images, the RMS roughness increased slightly and was higher than that on a glass substrate. Figure S3 shows the water contact angles on the epoxy substrate before and after the UV/O3 treatment. The water contact angle after the treatment decreased from 95° to 70°, which demonstrated an effect of the UV/O3


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treatment. The XPS analysis (Figure 1c) revealed the presence of π electrons; satellite peaks derived from the π electrons were observed in the 289–292 keV range. Considering that the base layer was smooth unlike that of SLIPS, the presence of the π electrons probably play an important role in immobilizing a lubricant layer.

Figure 1. Surface analysis. (a) SEM image of the TMOS-free base layer. (b) AFM image of the TMOS-free base layer demonstrating the surface smoothness with a RMS of < 5 nm. The AFM scanning range is 3×3 µm. (c) XPS analysis of the C1s of TMOS-free base layer for confirmation of the presence of π electrons.

Comparison of the wettability between the TMOS-free base layer and TMOS base layer. In our previous study, we fabricated a base layer of SPLASH using TMOS. In this study, we fabricated the base layer without using TMOS and increased the density of the π electrons to enhance the OH–π interactions between the lubricant layer and base layer. By making the interactions stronger, we needed to intensify the oleophilicity. To improve the stability in boiling hot water, we considered that the lubricant contact angle on a base layer should be lower than 10°. Figure 2a and 2b show schematic images of the TMOS-free base layer and the TMOS base layer, respectively. In general, TMOS is used as a spacer silane for making space and improving


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the mobility of the molecular chains of liquid-like surfaces.23,24 Therefore, it is inferred that the surface of the TMOS-free base layer has a higher density of phenyl groups than that of the TMOS base layer. The contact angles of canola oil, olive oil, sesame oil, grape seed oil, linseed oil, and oleic acid on these two base layers are summarized in Figure 2c. These oils are biocompatible because they are all edible oils; thus, we selected them as lubricant candidates. Their main ingredients are carboxylic acids. The –COOH groups attract the π electrons in the base layer; therefore, we expected that they could be used as lubricants.28 As a representative, we calculated the Hückel charge of an oleic acid molecule using the extended Hückel method and confirmed a slightly positively charged hydrogen atom (Figure S4 and Table S1).31 As shown in the schematic diagram (Figure S5), this positively charged hydrogen atom should be attracted to the π electrons through OH-π interactions. As shown in Figure 2c, all lubricant contact angles on the TMOS-free base layer were approximately 10° lower than those on the TMOS base layer. This result demonstrated that TMOS-free base layer had better wettability with the lubricants. We considered that the reason the wettability improved was because the number of phenyl groups per unit area, that is, the density of π electrons, increased, which strengthened the OH–π interactions. This assertion is supported by Figure 2d–f and Table S2–3. The water contact angles and water sliding angles on each base layer are shown in Figure 2d and 2e, respectively. Although there was little difference in water contact angles between the base layers, the water sliding angle on the TMOS-free base layer was approximately 10° higher than that on the TMOS base layer. As mentioned above, the TMOS enhances the mobility of surface molecular chains through working as a spacer.23,24 Therefore, the higher water sliding angle on TMOS-free base layer was caused by the higher phenyl-group density, which probably led to poor mobility of the surface phenyl groups. In addition, we analyzed these two kinds of base layers by XPS. The XPS


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result showed that the peak intensity of the π–π satellite of TMOS-free base layer was bigger than that of TMOS base layer (Figure 2f). To analyze this more quantitatively, we investigated the π–π satellite peak intensity, the C=C bond peak intensity, and the ratio of the intensities of the π–π satellite peak and the C=C bond peak in both base layers (Table S2). The ratios were calculated as 0.188 in the TMOS-free base layer and 0.0508 in the TMOS base layer; the π–π satellite peak intensity in TMOS-free base layer was 3.7 times higher than that in TMOS base layer. Therefore, the XPS result also indicated the increase of π-electron density. Moreover, we calculated the surface tensions of each base layer by using the Van Oss method.32 The contact angles of test liquids and the calculated surface tensions (γs), their apolar (γsLW) and polar components (γsAB) are summarized in Table S3. The surface tension of the TMOS-free base layer was 1.6 mN/m higher than that of the TMOS base layer. This phenomenon was mainly caused by an increase of the polar component γsAB, which probably indicated higher π-electron density.


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Figure 2. Comparison of the TMOS-free base layer and TMOS base layer. Schematic images of the TMOS-free base layer (a) and TMOS base layer (b). (c) Contact angles of the lubricants on TMOS base layer and TMOS-free base layer. Comparison of (d) the water contact angle and (e) the water sliding angle of the two types of base layers. (f) Comparison of the XPS results of the two types of base layers demonstrating the difference in the peak intensity of the π–π satellite peak.

Sliding properties. The water sliding angles of the surfaces prepared with the edible oils as lubricant layers are shown in Figure 3a. The surface tensions, the viscosities, and the densities of the oils are summarized in Table S4. When sesame oil or oleic acid were used as lubricant, they were


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uniformly spread on the base layer and the sliding angles were very low, 1° and 2°, respectively. The reason that the other lubricants did not show such a low sliding angle was because the water droplets sank into the lubricant layer and attached to the base layer. In addition, Smith et al. revealed that the sliding velocity of a droplet v is inversely proportional to the dynamic viscosity µ of the lubricant, that is, v ∝ 1/µ.34 As shown in Table S4, oleic acid has a lower viscosity than sesame oil. Thus, we selected oleic acid as the lubricant layer to achieve higher sliding speed. As shown in Figure S6, a dyed water droplet slid on the transparent sample after oleic acid lubrication. Both the TMOS-free base layer and the sample with the lubricant layer showed a high transparency of more than 90% in the range from 400–700 nm (Figure S7). In addition, their HAZE value and diffusion value were both less than 0.5% (Figure S8), which resulted from the smoothness of the base layer and lubricant layer. Figure 3b shows the relationship between the sliding speeds of a water droplet and tilting angle of the sample. From a practical usage perspective, the sliding speed is as important as the sliding angle. The sliding speed at 5° was 1.1 mm/s and 52.0 mm/s at 50°, which we assumed to be the approximate angle at which we drink a hot drink from a can. The sliding angle increased with the tilting angle.


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Figure 3. Sliding properties. (a) Water sliding angles of the edible oil lubricants. (b) Sliding speeds of a 10 µL water droplet at each tilting angle. The inset shows a magnification of the results at a low tilting angle.

Hot water repellency. We measured the relationship between the temperature of a water droplet and the water contact angles or water sliding angles (Figure 4). The infrared images inserted in this graph were captured just after a water droplet at each temperature fell on the sample. As shown in this figure, the contact angles were approximately 70° at all temperatures, and in particular, the surface repelled a hot water droplet with a temperature of 80°C at low sliding angles less than 2°. Supporting Movie 1 shows the sliding behavior of hot water droplets dyed by methyl orange, which was captured by a digital camera (side view) and a thermal camera (top view) to check the temperature at the same time. The supporting movie 1 indicated the hot water droplet that had the highest temperature of approximately 90°C slid off completely. In general, the vapor around the water droplet on a rough coating such as a superhydrophobic coating soaks into the rough structures and causes the Wenzel transition.1 In contrast, thanks to smoothness of the lubricant layer, the fabricated surface was hard to be affected by vapor. Therefore, the hot water droplet


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could slide off the fabricated surface at a very low sliding angle. However, we found that the sliding speed of water droplets on the room-temperature surface decreased with increasing the droplet temperature (Figure S9). It is considered that probe water droplets with a high temperature induced a temperature gradient, in other words, led to a surface-tension gradient in the lubricant layer. This surface-tension gradient caused Marangoni convection, which possibly made the lubricant layer wavy, as shown in Scheme S1.35 Therefore, the rough lubricant layer is considered to impede the movement of the water droplets. The effect on the sliding speed by Marangoni convection should be much smaller, if the surface temperature is close to the droplet temperature.

Figure 4. Hot water repellency. Relationship between the temperatures of a droplet and the water contact angles or water sliding angles. The inset images show water droplets at each temperature just after they fell on the sample.

Boiling and Retort tests. We investigated the stability in boiling hot water, as summarized in Figure 5. In this test, we utilized an epoxy substrate that is widely used in commercial coffee cans. A schematic image and


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a picture of actual settings for the boiling and retort tests are shown in Figure 5a or Figure S10, respectively. As shown in Figure 5a, the samples with red-dyed lubricant were boiled or retorted for 30 min. Figure 5b shows the base layer on the epoxy substrate (left) and the sample on which oleic acid dyed by oil red was introduced by spin-coating (right). The images in Figure 5c show the appearance of the samples after the boiling test (left two images) or the retort test (right two images) for 30 min. As shown in Figure 5c, the TMOS-free base layers were used in the top two images, and the TMOS base layers were used in the bottom two images. In the case of the TMOS-free base layers, the original uniformity of the red-dyed lubricant layers was maintained throughout the whole area after both the boiling test and the retort test. Conversely, in the case of TMOS base layers, the original uniformity of the lubricant layer was lost. The reason why the TMOS-free base layer led to a uniform lubricant layer after the boiling test and the retort test was that the higher π-electron density of the TMOS-free base layer strengthened the OH–π interactions compared with that of the TMOS base layer. Then, we investigated the water contact angles and the water sliding angles both before and after the boiling and retort tests (Figure 5d). Importantly, since the uniform lubricant layers were kept through the boiling and the retort test, the original sliding angle was maintained and it rather slightly decreased. Furthermore, the water contact angle increased from 60° to approximately 70° after the tests. We inferred that the reason that the changes of the sliding angle and the contact angle happened after the tests was because of the effluence of the components in the lubricant except for oleic acid. The purity of the oleic acid used in this study was approximately 85%. Therefore, the other components, which did not interact with the π electrons through OH–π interactions and were probably less hydrophobic, were removed by the boiling water. Hence, the changes were occurred after the tests. In addition, we measured the water sliding speeds at 10° tilting angle both before and after the tests (Figure


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5d). Sliding speed changes between before and after the tests were less than 15%, which indicated stable sliding property. The small decreases of the sliding speeds were probably caused by decrease of the lubricant layer thickness after the boiling and the retort test, even though the uniformity of the lubricant layer was kept. The decreases should be small if lubrication by spin-coating is conducted by a higher spin speed to get rid of residual lubricant.

Figure 5. Boiling and retort tests. Epoxy substrates were used in these tests. (a) Schematic image of the boiling and the retort test. (b) Images of the base layer coating before lubrication (left) and after lubrication (right). (c) Images after the boiling test and the retort test. The top two images correspond to the surfaces based on the TMOS-free base layer and the bottom two images


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correspond to the surfaces based on the TMOS base layer. (d) The water contact angles and water sliding angles of the substrates before the tests, after the boiling test, and after the retort test. (e) The sliding length over time and the calculated sliding speed of a 10 µL water droplet on the surface tilted at 10° before the tests, after the boiling test, and after the retort test.

Stability / Durability. For practical use as food packages, stability against solutions with a wide range of pH values is needed. The relationship between pH value and water contact angles or water sliding angles is shown in Figure 6a. Although water droplets with pH >12 could not slide off, the fabricated surface repelled water droplets with pH values less than 12 and exhibited sliding angles below 5°. The contact angle decreased gradually in basic solutions, which was attributed to the neutralization reaction between oleic acid and the basic solution. However, the pH range at which the surface can repel a water droplet is sufficient for application as food packaging. The relationship between the sample’s surface temperature and the water contact angles or water sliding angles is shown in Figure S11. The water contact angles were approximately 60° and the sliding angles were lower than 2° at 100°C or lower temperatures thanks to the thermal stability of the base layer and the high boiling point of the lubricant layer (oleic acid).36 However, a uniformity of the lubricant layer was lost over 100°C through dewetting, which resulted in an increase of the water sliding angle. Figure 6b and 6c show the thickness of the lubricant layer (Figure 6b) and water sliding angles (Figure 6c) after adding shearing stress by spinning with a spin coater from 0 to 6000 rpm for 30 s. Although the thickness decreased with increasing spin rate, the coating maintained a sliding angle below 5°. These results indicated that the immobilizing mechanism through OH–π interactions was durable against shearing stress.


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However, stability against flowing water for a long term was not achieved, as shown in Figure S12. This deterioration was caused by loss of the lubricant layer, since the OH-π interactions were not durable against continuous flowing water. Although the water sliding angle converged at that on the TMOS-free base layer after flowing water for 15 min, it was recovered by casting a new lubricant layer. The result of the evaporation residue measurements is summarized in Table S5. The evaporation residue was not observed after TMOS-free base layer formed on the epoxy substrate had been immersed in heptane at 25°C for 1 h. This result indicated that the components of the base layer did not come off. Therefore, the components of the base layer coating do not enter the human body, and thus, this surface should be suitable for food package coatings.

Figure 6. Stability and durability. (a) The variation of contact angles and sliding angles with pH value. (b) The thickness of the lubricant layer and (c) the water sliding angles after adding shearing stress with a spin-coater for 30 s.

CONCLUSION In this work, we fabricated a slippery surface that immobilized a biocompatible lubricant layer on a smooth base layer through OH–π interactions. The wettability measurements of two types of base layers with different π-electron density revealed that the TMOS-free base layer displayed


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better affinity between the lubricant layer and the base layer thanks to a higher π-electron density. The control of the π-electron density enabled the lubricant layer to be immobilized on the smooth base layer through both boiling and retort tests, which resulted in improved boiling stability. This type of lubricant has been difficult to be immobilized because of its high surface tension compared with other lubricants used for LISs. Because of the smoothness of the lubricant layer, the resultant surface demonstrated repellency of water at temperatures between 20 and 90°C. We believe the development of this surface provides new scientific insights into antiwetting liquid surfaces under high-temperature environments, and is also useful in wide variety of applications.

ASSOCIATED CONTENT Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org. A scheme of our hand-made instrument to produce hot water droplets; SEM and AFM images of both the bare epoxy substrates and the same substrates after TMOS-free base layer coating; Influence of UV/O3 treatment on water contact angle; Hückel charge distribution; Schematic diagram of OH-π interaction; Comparison of the π-π satellite peak intensities; Surface tensions of the base layers; Characteristics of the lubricant candidates; Time-lapsed images showing a sliding water droplet; Optical properties; Relationship between the temperature of water droplets and the sliding speed; A scheme showing the difference of sliding behaviors between room temperature water and high temperature water; A photograph of the boiling and the retort tests; Relationship between the surface temperature and the water contact angles or the water sliding angles; A schematic image and a result of water flowing test; Results of evaporation residue measurement; (PDF)


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Supporting Movie 1 showing sliding behavior of hot water droplets and their temperatures at the same time (AVI)

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] Present Addresses †

Center for Material Design Science, School of Integrated Design Engineering, Keio University,

3-14-1 Hiyoshi, Kohoku-ku, Yokohama, Kanagawa 223-8522 Japan Author Contributions R. T. proposed the research, designed the experiments, collected and analyzed the data. R. T., M.T., T. Matsubayashi, and T. Moriya collected the data. R. T. wrote the paper. M. T., T. Matsubayashi, and K. M. commented on the manuscript. S. S. supervised the project. All authors discussed the data, and substantially contributed to the research. Funding Sources This work was supported by JSPS KAKENHI (grant number JP16J06070, JP26420710). Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT We are deeply grateful to Dr. Kouji Fujimoto whose meticulous comments were of enormous help. We also thank Ryohei Yoshikawa for providing impressive schemes. We express our


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sincere gratitude to Prof. Walter Navarrini in Politecnico di Milano, Prof. Nicole Zacharia and Prof. Bryan Vogt in The University of Akron for useful discussions. We would like to thank the Daiwa Can Company for their assistance in the evaporation residue measurement. M. Tenjimbayashi thanks predoctoral fellowship (DC1) from Japan Society of Promotion of Science (JSPS).

ABBREVIATIONS LIS, liquid-infused surfaces; SLIPS, slippery liquid-infused porous surfaces; SPLASH, surfaces with

π-interaction

liquid

adsorption,

smoothness,

and

hydrophobicity;

PTES,

phenyltriethoxysilane; TMOS, tetramethoxysilane.

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“Table of contents only”


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A Fluorine-free Slippery Surface with Hot Water ... - ACS Publications

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